Hydraulic fracturing with overflushing



""HYDR UL orRAcrumNG WITH r a OVERFLUSHING 'Donald .Trott, 'Dallas,-Tex., assignor to California 'Research Corporation,- San Francisco, Calif., a corporation of Delaware 7 NoDrawing. Origiiial application September 8;1953,

SerialN0. 379,077; Divided antl this application April 1-25, 1957, Serial No.1 655,018

3Claims; (Cl. 166- 22) This invention relates in general. to thetreatment of oil and gas wells, and relates in particularto the treatment of such wells toincrease the production therefrom by hydraulic fracturing of the formations therein.

- It is well known in the art of well stimulation to inject a.fluid into aformation and apply pressuretothe fluid w to fracture the: formation. The fluid used may be a low penetrating fluid or gel which is subsequently reacted with a. solvent therefor to facilitate removal of thefluid from the Well, or the fluid may be a liquid of lower viscosity which acts against an underreamed portion of the formation-to produce fracturing. Spacer objects, such as sand, may be added to the fracturing fluid to-act as propping agents in maintaining the'fractures open after release of the fracturing pressure.

After fracturing by these methods, the fractured formation is overflushed with a large volume, usually equal to the volume of fluid used during fracturing, of a low gravity oil at a pressure and injec- -tion rate usually equal to the pressure used during fracturing. This overflush is designed to carry the sand into i the formation away from the well bore to prevent large quantities of sand from entering the pumps upon resumption of production from the formation.

The above prior art methods have the'disadvantage that the pressures and injection rates in the overfiushing operation are so high as to carry away substantial quantities of the propping sand from the critical permeability 'zoneadjacent the-well bore, resulting in an undesirable decrease in the artificially formed effective permeability of the formation.

It has also been proposed to utilize particles of abrasive material such as sand in a fluid carrier to abrade the pas- -sages of a producing formation to thereby increase the permeability thereof. The particle size is progressively wincreased during the .treatment and the direction of flow is :periodically-reversed toincrease the abrasive action.

:However, thismethod has the disadvantage thatit relies I primarily on the abrasive action of the particles, rather than-hydraulic fracturing, to produce anincrease in the effective permeability of the formation, "withthe result thata considerable amount of time-and abrasive material 1 is-required to produce a substantial'increase in theeffect ive formation permeability.

. .These prior art methods have the further disadvantage v that-the;,concentration, or amount per unit'volume, of -sandin the fracturing fluid is maintained substantially constant. throughout the fracturing operation.

discovered that itis desirable that the concentration of 1 sand in the fracturing fluid at the start of the operation be low to permit the fracturing fluid to more easily penetrate the formation passages and start the fractures,

I have then progressively increase the sandconcentration to permit gradual enlargement of the fractures, and to have a high-concentration of sand in the fracturing fluid near the end of the fracturing operation to insure the deposition 1 of a-large quantity of sand to fill and prop open all of the fractures developed, particularly in the critical permeability zone adjacent the well bore United States Pa ice 2,859,819 l 'atentedNov. 11-, 1358 Broadly, the present invention contemplates a method of increasing-well productivity by hydraulic fracturing of the production formation, in which spacer objectsare added. to the fracturing liquid, and in which an-overflushing fluid is applied to the fractured formation at a relatively low pressure and injection rate to carry away farther into the formation and to re-establish a low viscosity fluid in the criticalpermeability zone adjacent the well bore.

It is therefore an object of this invention to provide anlimproved method of hydraulically fracturing a sub- I terranean formation.

It is a further object of this invention to provide a method of increasing the productivity of a formation penetrated by a well bore by hydraulically fracturing theformation with aliquid having spacer objectssuspended therein and applying a high viscosity liquid to the fractured formation at a reduced pressure and injec- "tionrate to remove fracturing debris from adjacent the well bore. I It is' an additionalobject of the present invention to provide a method of increasing the productivity of a petroliferousdormation penetrated by a well'bore in whichthe formation is fractured by a fracturing'liquid having spacer objects suspended therein, then the fractured formation is overflushed with a high viscosity liquid at a reduced pressure and injection rate to remove I fracturing debris from adjacentthe well bore, and then the fractured formation is overflushed with a low viscosity fluid to re-establish a low viscosity fluid in theportion of the formation adja'centthe well bore.

The fluid utilized in the fracturing operation may be of any suitable type, such as a low penetrating fluid or gel. However, I have found that ordinary crude oil,

often available at the Well site, is very-suitable. --The fracturingfluid is placed in a well, at 'theelevation of a producing formation. Packers may be, andpreferably are, used toisolate and confine the section of the well to be fractured. Thefracturing fluid may be 'preceded by a smallquantity of light oil which ispu'rnped into the formation at alhigh pressure, such as 2000 p. s. i';, to insure that the perforations in the' formation'a're open for passage of the fracturing mixture ther'ethrough.

The fracturing fluid may then beint'roduced into the Well; and progressively increasing amounts'of' suitable spacer objectsper: unit volume of fluid are addedto the fluid. :Suitable spacer objects which may-be used are gravel, glass, iron and steel, but preferably sand is utilized: because of its low cost and ready "availability.

.This mixture of fracturing fluid'and progressively increasing concentrations of spacer objects is pumped into thezone tobe' fractured at a suitablerate and under a suitable pressure. By using a. low concentration of spacer objects in thefracturing fluid at the startof the operation, the fracturing commences more easily, apparently because of the easier penetration of the fracturing fluid with itslow spacer object content. After the fractures have been thus started, the spacer object content of the fracturing fluid is increased, to causei the fractures thus developed to open' further in a gradual or stepwise fashion. Near the. end of the fracturing operation, the concentration of spacer objects in the fracturvtion. I r

' I, .After the fracturing pressure has bled off into the .formation, the formation may be treated with an overflush of a large volume, preferably at least equal to the ing fluid is high to insure filling and propping open all the fractures developed during the operation and to leave a heavy load of spacer objects in the critical per meability zone immediately adjacent the well bore. The

range of concentrations of the spacerbjects in the fluid during the course of the fracturing ,may vary from 4" pound. pergallon of fluidpto 5 pounds per gallon of fluid. Theoccurrence of the fracturing may 'be indicated by a drop or break in the pressure of the injected fracturingv mixture. After fracturing, a small volume of high viscosity oil may be injected into the well to insurethat substantially all of the spacer objects have entered the formation, since it appears that in' some cases the spacer objects are not displaced as rapidly as r the fracturing 'fluid in which they are suspended;

7 As soon as the last of ithe mixture of spacer objects an d -fractu'ring fluid has cleared the well bore I and "gone into the formation, as measured by displacement or other-suitable means, the pumps supplying the fractur ing mixture are slowed down or stopped and the well is sealed in, permitting the fracturing pressure to bleed off into the formation. This step is distinctly different 1 fromprior fracturing methods, in which, upon completion of fracturing, an overflush of a low viscosity fluid is pumped into the fractured formation at as high a pressureand injection rate as possible.v I believe that the high pressure and high injection rate are undesirable :because." they tend to carry away the deposited spacer objects from the critical permeability zone adjacent the well bore, thus permitting some of the fractures in the critical zone to close and thereby reduce the effective permeability of the formation. the pumps as soon as the last of the spacer objects are cleared into the formation,-the fractures in the critical zone are propped wide open and the propping objects are permitted to remain in place in the critical zone, resulting in a high effective permeability of the'formavolume of'fracturing fluid, of a high viscosity fluid. This I ,overflushing isdone at a slow injection rate, preferably not over barrel per minute, and at as low a'pressure as results from the slow injection rate, but preferably not; over /2 of the average fracturing pressure.

. This overflush of heavy fluid is designed to clean the formation, near-the well bore and onout into the formation,

By slowing or stopping of particles of material which have been loosened from theformation during the fracturing'operation, and to carry these particles as far from the well bore as possible. A high viscosity fluid is preferred because its greater suspending'power results in more complete removal of these loose particles. I believe thatthe abovementioned particles, by their deleterious bridging action in the developed fractures, are responsible for the rapid declines in production rates often observed with the prior art fracturing methods. The useof the low pres sure and injection rate mentioned above in this stage is important to'avoid carrying away into the formation the heavy load of spacer objects which has'been deposited in the critical permeability zone of the formation.

The propping spacer objects are wedged into the fractures and hence are capable of withstanding greater pres- I sure than the loose particles which it is desired to remove. In connection with the above steps of reducing the formation pressure after fracturing and overflushing with a high viscosity fluid, it will be seen that the eflicacy of these steps is not dependent upon the use of progressively increasing amounts of spacer objects during fracturing and that each of these steps independently is an improvement over the prior art fracturing methods.

The above overflushing operation may be followed by the application to the formation of a low viscosity 011 a) displace the above-described 10m particles even farther into the formation and to re-establish a low f viscosity fluid in the critical permeability zone of the formation to aid in obtaining production from the formation.

As an example of the effect of my invention on-the operation of a representative well, the following data to this invention.

of 1%" barrels per minute;with *no apparenti 'pressure break or drop during this pumping. This phase of the treatment was to insure that the perforations in the formation were open for passage of the fracturing mixture.

This was followed by a mixture of 1000 gallons of 25 gravity oil and 500 pounds of sand, giving a'concen'tration of /2 pound of sand per gallon. This mixturewas pumped into'the formation at a rate of approximatelvP/a barrels per minute.

same rate of 1% barrels per minute. tion of sand was then increased to 1% pounds per gallon by mixing 500 pounds of sand with 400 gallons of the 25 gravity oil and this mixture. was

pumped into the formation at 1% barrels per minute. i

During the pumping of the above mixtures, the 'pres-- i sure increased from 2000 p. s.i. to 3000 p. s. i and then 2' dropped to 2700 p.'s. 1'. near the end of thetre'atment, i i probably indicatin the occurrence of the fracturing.

The mixture of sand and oil was followed by the injection of'3 barrels of heavy oil at 2700 p. s. i. to insure that all of the sand had cleared the hole and had entered the formation.

After completion of.the above pumping, the pumps were shut down and the pressure in the formation was- 1 V permitted to bleed off to 1000 p. s. i. Then 14 barrels of heavy oil were injected at a rate of barrel per minute to carry away from the well bore the particles loosened during the fracturing operation.

the same injection rate, to carry the loose particles farther out into the formation and to re-establish a lowviscosity oil in the portion of the formation adjacent the well bore. During the application of these heavy oil and light oil overflushes, the surface injection pressure ranged from 1000 p. s. i. to 1200 p. s. i.

After being thus treated, the well produced on consecutive days, after recovery of the treating-oil, the fol-.

lowing ,quantities of oil: 118, 131, 125, 118, 102, 105, 116,102, 122, 115, 102, 115, 120, 102, 127, 100, 100, 108, 10 4, 108, 100, 104, 114, 112, 113, 120, 113, 115, and 120 barrels per day with no stantial decline apparent. It is this relatively small decline in production whichis believed to be the biggest v advantage of the present invention over the 'prior art well stimulation methods. In wells treated by the method of this invention, the largest decline in production'at the I 1 end of the first month after treatment has been 20'percent; whereas, in wells treated by the prior art fracturing methods, declines in production of 60 percent during the first month after treatment often occur. All of the declines mentioned are calculated after the usual flush production, which lasts a few days immediately following this 7' type of treatment.

An example of this is shown in the case of'two additional wells. One well had been producing 27 barrels of oil per day when it was treated by the method of this invention, substantially as described above. 1 After such treatment, the well produced 91 barrels of'oil'per day,

Then, the concentration-of sand i i was increased to pound per gallon by mixing f500 pounds of sand with 750 gallons of the 25 gravit'v oil, and this mixture was pumped into the formation atthe The concentra- Thish'eavy oil overflush was followed by 3 barrels of light 'oil at sub excluding the first two days of flush production immediately after the treatment. At the end of the first month after treatment, this well was producing 75 barrels of oil per day, representing a decline of only 18 percent, and at the end of four months after treatment, the well was producing 67 barrels of oil per day, representing a decline of only about 27 percent.

In contrast to this low decline, another well in the same field had been hydraulically fractured in accordance with the prior art method, wherein a constant concentration of sand was used and the overflushing done at a high pressure and injection rate. The well had been producing 8 barrels of oil per day prior to treatment, and immediately after treatment and after recovery of the flush production, the production increased to 47 barrels of oil per day. However, at the end of the first month after treatment, production had declined to 20 barrels of oil per day, representing a decline of almost 58 percent. At the end of the second month, production had further declined to 13 barrels per day, representing a total decline since the treatment of 72 percent. Because of this decline, the well was plugged back and fractured in another sand by the prior art method, increasing the production to 99 barrels per day. However, the production declined 42 percent during the first month after this treatment and production further declined at the end of the second month to 43 barrels per day, a total decline since the second treatment of 56 percent. Although it is realized that the above two wells are not exactly comparable, it is believed that since both wells are in the same type reservoirs in the same field, the results are indicative of the sustained improved production obtainable through use of the well stimulation method of this invention.

This application is a division of my copending application Serial No. 379,077,-filed September 8, 1953, entitled Method of Increasing Permeability of Subterranean Formations by Hydraulic Fracturing.

Although but a few embodiments have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. The method for increasing the productivity of a subterranean formation penetrated by a Well by hydraulic fracturing comprising the steps of pumping, under increasing pressure, a fracturing liquid having sand suspended therein into a confined zone in said well until said fracturing occurs, maintaining a decreased pressure on said fracturing liquid after fracturing occurs, subsequently injecting into said formation at a pressure and an injection rate no greater than one-half the average pressure and the average injection rate used during said fracturing a first overflushing liquid having a viscosity suflicient to carry away from the portion of said formation adjacent said well particles abraded loose during said fracturing, and then injecting into said formation at a pressure and an injection rate no greater than one-half the average pressure and the average injection rate used during said fracturing a second overflushing liquid having a viscosity appreciably lower than said first overflushing liquid to displace said first overflushing liquid from the portion of said formation adjacent said well bore.

2. The method for increasing the productivity of a subterranean formation penetrated by a well by hydraulic fracturing comprising the steps of pumping a fracturing liquid under increasing pressure into a confined zone in said Well until said fracturing occurs, reducing the pressure of said liquid substantially below the. pressure at Which said fracturing occurred, maintaining the reduced pressure on said liquid for a period of time, subsequently injecting into said formation at a pressure and an injection rate substantially below the average pressure and the average injection rate used during said fracturing a first overflushing liquid having a viscosity suificient to carry away from the portion of said formation adjacent said well particles abraded loose during said fracturing, and then injecting into said formation at a pressure and an injection rate no greater than one-half the average pressure and the average injection rate used during said fracturing a second overflushing liquid having a viscosity appreciably lower than said first overflushing liquid to displace said first overflushing liquid from the portion of said formation adjacent said Well bore.

3. The method for increasing the productivity of a subterranean formation penetrated by a well by hydraulic fracturing comprising the steps of pumping a fracturing liquid under increasing pressure into a confined zone in said well until said fracturing occurs, reducing the pressure of said fracturing liquid in said confined zone, injecting into said formation at a pressure and an injection rate in the order of one-half the average pressure and the average injection rate used during such fracturing a first overflushing liquid having a viscosity suflicient to carry away from the portion of said formation adjacent said well particles abraded loose during said fracturing, and then injecting into said formation at a pressure and an injection rate in the order of one-half the average pressure and the average injection rate used during said fracturing a second overflushing liquid having a viscosity appreciably low-er than said first overflushing liquid to displace said first overflushing liquid from the portion of said formation adjacent said well bore.

References Cited in the file of this patent UNITED STATES PATENTS 2,596,843 Farris May 13, 1952 

1. THE METHOD FOR INCREASING THE PRODUCTIVITY OF A SUBTERRANEAN FORMATION PENETRATED BY A WELL BY HYDRAULIC FRACTURING COMPRISING THE STEPS OF PUMPING, UNDER INCREASING PRESSURE, A FRACTURING LIQUID HAVING SAND SUSPENDED THEREIN INTO A CONFINED ZONE IN SAID WELL UNTIL SAID FRACTURING OCCURS, MAINTAINING A DECREASED PRESSURE ON SAID FRACTURING LIQUID AFTER FRACTURING OCCURS, SUBSEQUENTLY INJECTING INTO SAID FORMATION AT A PRESSURE AND AN INJECTION RATE NO GREATER THAN ONE-HALF THE AVERAGE PRESSURE AND THE AVERAGE INJECTION RATE USED DURING SAID FRACTURING A FIRST OVERFLUSHING LIQUID HAVING A VISCOSITY SUFFICIENT TO CARRY AWAY FROM THE PORTION OF SAID FORMATION ADJACENT SAID WELL PARTICLES ABRADED LOOSE DURING SAID FRACTURING, AND THEN INJECTING INTO SAID FORMATION AT A PRESSURE AND AN INJECTION RATE NO GREATER THAN ONE-HALF THE AVERAGE PRESSURE AND THE AVERAGE INJECTION RATER USED DURING SAID FRACTURING A SECOND OVERFLUSING LIQUID HAVING A VISCOSITY APPRECIABLY LOWER THAN SAID FIRST OVERFLUSING LIQUID TO DISPLACE SAID FIRST OVERFLUSING LIQIUD FROM THE PORTION OF SAID FORMATION ADJACENT SAID WELL BORE. 